1. Field of the Invention
The present invention relates to an apparatus and method of curtain coating for continuously applying onto a running belt-shaped substrate (hereinafter referred to as a “web”) a coating solution in the form of a thin film.
2. Description of the Related Art
There are various known coating apparatus and methods that involve continuous application of coating solution onto a running web surface, with representative examples of coating method including, for example, blade coating, roll coating, wire bar coating, die coating, and curtain coating. The production of materials that require high coating speed (e.g., thermosensitive recording materials, magnetic recording materials, inkjet recording sheets and silver halide photographic photosensitive materials) often employs curtain coating.
The curtain coating method or apparatus includes the steps of discharging through slits coating solution that has been supplied in the manifold of the coating head, retaining a formed thin-film liquid (hereinafter referred to as a “curtain film” or “curtain”) at the curtain edge guides that are provided at both sides of the coating head so as to face the coating surface of the web, and allowing the curtain film to fall down onto the running web surface to thereby form a coating film that covers the web surface.
When the curtain width is to be made equal to or less than the web width upon curtain coating that applies a curtain of coating solution onto a coating surface of the web in a gap formed between the curtain edge guides and coating surface, the coating solution is drawn to the center of the coating film in the width direction. This leads to a so-called “neck-in” phenomenon in which excessive deposition of the coating solution occurs at the edge of the coating, as shown in FIG. 1. Note in FIG. 1 that reference symbol 1 denotes an excessive deposit at the coating edge, reference symbol 2 denotes a coating, and reference symbol 3 denotes a web.
This excessive deposit causes dry process failure in the subsequent drying step conducted using a dryer, leading to stains on the web transporting roll after passing through the dryer and/or on the edges of the backside of the web when it is taken up. Even when the excessive deposit has been fully dried in a drier, the take-up unit for coated paper provided in the coater takes up the coated paper with the position of the excessive deposit being at the same level in the roll. This leads to unwanted thickening or raised portions in the roll where the excessive deposit is formed, leading to web breakage.
In an effort to avoid generation of such excessive deposits at the coating edges, Japanese Patent Application Laid-Open (JP-A) Nos. 2000-513, 2000-218209, 2001-104856 and 2005-512768 each disclose a method of making the flow rates of the coating solution at opposite sides of the slide surface close to the flow rate at the center of coating solution by feeding an auxiliary solution along the opposite edges of the slide surface, but each method requires a large amount of auxiliary solution because it is fed along the guide edges. For this reason, the auxiliary solution becomes more likely to be mixed with the coating solution fed along the edge guides, or the auxiliary solution flow rate become uneven along the edge guides; therefore, stable deposition amounts cannot be ensured at the coating edges, leading to defective products. Moreover, there is a drawback that these methods require a complex coater.
JP-A No. 2000-254567 discloses a method of removing excessive deposits at the lower edges of curtain edge guides by suction. This method, however, can remove excessive deposits only when the suction nozzles are placed in contact with or in close vicinity of the excessive deposits. To achieve removal it is required to make the distance between the suction nozzle and web considerably small, but this causes dusts attached on the running web surface to get stuck on the nozzles and triggers web breakage.
JP-A No. 2004-16877 discloses a coating method that applies a coating solution on a web while creating uncoated portions on both sides of the web by making the curtain width larger than the web width and by folding both sides of the web at the upstream from the position where the curtain collides with the web surface. Although this method entails no generation of excessive deposits at the coating edges indeed, the curtain needs to be larger in width than the web, and in addition, portions of the coating solution that exceed the width of the web are not applied onto the web. In the case of a single layer coating, however, there is no problem since the coating is formed using one coating solution and thus the coating solution can be reused. On the other hand, in the case of a multilayer coating formed of layers of different coating solutions, these coating solutions cannot be reused and should be discarded, significantly reducing the productivity.
Japanese Patent Application Publication (JP-B) No. 06-91979 discloses a coating method that prevents excessive deposits formed at both sides of the curtain coating from being in contact with the web by making the web width smaller than the lip tip width. In this method, however, portions of the coating solution exceeding the width of the web fail to be applied onto the web; therefore, this coating method has the same technical problem as the method disclosed in JP-A No. 2004-16877.
Another approach to overcome the above-mentioned problem is to remove, by means of vacuum, portions of coating solution that have been excessively deposited at the edges of the coating after deposition of the coating on the web, but it is difficult to remove only such excessive deposits at the edges and it is often the case that it results in unwanted removal of portions of the coating solution that are close to the center of the coating in its width direction.